Recent findings in Drosophila have shown that 1) a significant proportion of spontaneous mutations are caused by insertions of mobile genetic elements, and 2) certain genetic suppressor systems are mediated through insertions of specific mobile elements. We are investigating the molecular mechanism of one such suppressor system: recessive mutations at the suppressor-of-sable [su(s)] locus suppress recessive mutations at the vermilion (V) locus that are caused by insertions of the mobile elements 412 and B104. DNA sequences of su(s) have been cloned and are being characterized to determine the transcription orientation and the locations of the control and coding sequences. A primary goal of the work is to determine the protein product of su(s), a reduction or an absence of which effects suppression of several v alleles. Coding sequences from su(s) will be ligated into an expression vector to produce a fusion protein, against which antibodies can be produced. Antibodies against the su(s) portion of the fusion protein will be recovered and used as probes to identify the location and function of the su(s) protein product within the organism. The interaction of the su(s) protein with the v locus will be studied to determine how suppression is effected. By gaining an understanding of this phenomenon, we will learn if this type of suppression mechanism is an adaptive feature of Drosophila to deal with mutations caused by mobile element insertions.